add a parameters string to MUSIC_Init, improve fluidsynth driver

The parameters string is parsed by each driver, but the idea is
to keep the format "name=value name=value" where possible.
Also, the fluidsynth driver will scan a list of known paths for
a soundfont file to use.
add functions to get the current driver id/name for fx and music

Useful when auto detect was used for initialisation
update Core Audio driver to 10.6 framework
finish Core Audio driver with sound and music

The driver now uses the Audio Graph infrastructure to mix PCM and
MIDI synth. This driver now supersedes the SDL driver on OS X.